This invention relates to an edge formation process for aluminum solid electrolytic capacitors.
Electrolytic capacitors with excellent high frequency characteristics are in high demand due to speed requirements of circuits for devices such as computers and wireless communications. In addition, high capacitance is required in the low voltage circuits that are used in these devices. Conductive polymers such as polypyrrole, polyaniline, polythiophene, and their derivatives, are finding increasing use as cathodes for electrolytic capacitors because such polymers have much higher conductivity than the liquid electrolytes and manganese dioxide cathodes currently used in these capacitors.
A wet electrolytic capacitor has an anode metal, a dielectric, a liquid electrolyte, and a cathode. Valve metals such as tantalum, aluminum, and niobium are particularly suited for the manufacture of high surface area electrolytic capacitors. The valve metal serves as the anode, and an oxide of the valve metal, coated by electrochemical oxidation of the valve metal surfaces, serves as the dielectric. The process of electrochemically coating a valve metal with a dielectric oxide is called formation. In order to maximize the dielectric surface area, and hence increase the volumetric efficiency of the capacitor, the valve metal substrates are porous bodies. These porous bodies can take the form of etched foils or slugs of compressed powder. The liquid electrolyte is impregnated into the porous body. A high surface area cathode completes the circuit. Etched aluminum foil is a particularly preferred anode material for wet electrolytic capacitors.
In the manufacture of wet aluminum electrolytic capacitors, the aluminum foil is etched to high surface area, coated with a dielectric oxide film, slit to the proper width, and then cut to length. During the slitting and cutting-to-length operations, the dielectric oxide on the edges of the foil is damaged and bare aluminum is exposed. The foil is then wound, placed in a can (along with the cathode), and filled with a non-aqueous fill electrolyte. The non-aqueous fill electrolyte is composed of, for example, borates in non-aqueous solvents containing a very small amount of water. After filling with electrolyte, the cans are sealed to prevent electrolyte from escaping and to keep additional water out.
A critical part of conditioning a wet aluminum electrolytic capacitor is repairing the damage to the dielectric oxide on the edges of the slit and cut-to-length foil and any damage to the dielectric oxide on the face of the foil that incurred during the winding operation. If these edges are not re-formed, the capacitor will have a high leakage current. The non-aqueous fill electrolytes, containing a very small amount of water, are very efficient in re-forming oxide on the edges.
In the manufacture of a solid aluminum electrolytic capacitor with a conductive polymer cathode, the foil etching, forming, and slitting are done in a similar manner to that of wet aluminum electrolytic capacitor. However, the conductive polymer is not efficient at re-forming a dielectric film on the slit and cut edges and at repairing damaged oxide on the face. Therefore, this must be done in a separate step before the conductive polymer is impregnated into the aluminum/aluminum oxide anode.
Re-forming the slit and cut edges can be accomplished by immersing the elements in a formation bath or a series of formation baths. The requirements for these edge formation baths are threefold: 1) They must form a high quality dielectric oxide on the cut edges, 2) They must repair any damage to the dielectric oxide on the face of the clement that was damaged during the slitting and cutting to length operation, and 3) They must not damage the dielectric oxide already on the face of the element. In addition, the formed dielectric oxide needs to have excellent hydration resistance.
Hydration resistance is critical for aluminum solid electrolytic capacitors with conductive polymer cathodes. After impregnation with the conductive polymer, the capacitors are washed extensively in water to remove excess reactants and reactant byproducts. This washing is at elevated temperature ( greater than 50xc2x0 C.). The aluminum oxide film is exposed to conditions very conducive to hydration during this washing process, and, therefore, the aluminum oxide film must have a high degree of hydration resistance. Hydration of the oxide during the washing process, or on subsequent storage after washing, can result in hydrated oxide in the weld zone and this hydrated oxide is difficult or impossible to weld through to make a good attachment to the lead frame.
A high degree of hydration resistance is also required during storage or use of capacitors in high humidity environments. If the oxide becomes hydrated during use, the capacitor leakage current will increase, or the capacitor can become a short circuit.
It was discovered that prior art electrolytes have deficiencies when used for edge formation of aluminum anodes intended for use in solid aluminum electrolytic capacitors with conductive polymer cathodes. The fill electrolytes used in wet aluminum capacitors are not suitable for use outside a sealed can because of their toxic nature and their propensity to adsorb water from the air. Thus they cannot be used in open, mass production electrolyte baths.
Electrolytes used for the production of the original aluminum oxide film are also not completely suitable because they are designed to form oxide on a freshly etched surface or a hydrated oxide surface and not designed to form oxide on cut edges and to repair oxide on the face (cf. U.S. Pat. Nos. 3,796,644; 4,113,579; 4,159,927; 4,481,084; 4,537,665; 4,715,936). In addition, compromises must be made in the selection of an electrolyte because of the high current efficiency needed to economically produce a dielectric oxide over the entire etched aluminum surface.
Slitting and cutting the foil to length mechanically damages the edges and this mechanical damage should be repaired before or during the formation of the dielectric oxide film on the edge.
Several electrolyte systems have been considered for the edge formation of aluminum electrolytic capacitors with a solid conductive polymer cathode. Low leakage current and high capacitance can be achieved by producing a thick, porous layer on the edge using aqueous solutions of oxalic acid, followed by forming a barrier layer with aqueous solutions of ammonium adipate (EP 1,028,441 A1). A flowchart of this prior art edge formation process is shown in FIG. 2. The parts are first anodized in oxalic acid, rinsed, and dried. This produces a thick, porous layer on the edge. Since oxalic acid has a low pH, it also tends to remove the very outer layers of oxide from the surface. The parts are then formed in ammonium adipate, rinsed, and dried. This step produces a dielectric oxide on the edge. This is followed by a depolarization step and another formation in ammonium adipate, rinse, and dry. The resulting films are unstable toward hydration. The hydration resistance of the preexisting dielectric oxide is impaired because of the attack by oxalic acid. Neither ammonium adipate alone or the oxalic acid-ammonium adipate system are capable of forming a hydration resistant oxide on the edges. This leads to problems with leakage current instability in production, welding of the capacitors to the lead frame, and long-term stability towards hydration. It is desirable to have an edge formation electrolyte system, which provides a product with a hydration resistant oxide.
It was discovered that edge formation in an aqueous citrate solution followed by formation in an aqueous phosphate solution imparts high hydration resistance to the foil and results in a minimal loss of capacitance.
The invention is directed to a process for edge forming a slit and cut-to-length foil having a dielectric oxide film on at least one surface comprising forming the foil in an aqueous citrate electrolyte, preferably an aqueous ammonium citrate electrolyte, depolarizing the foil, and forming the foil in an aqueous phosphate electrolyte, preferably an ammonium dihydrogen phosphate electrolyte. Using this formation process, a foil with excellent hydration resistance and capacitance is produced.
The invention is further directed to a process for edge forming a slit and cut-to-length aluminum foil having a dielectric oxide film on at least one surface comprising forming the foil in an aqueous ammonium citrate electrolyte, then depolarizing the foil, and then forming the foil in an aqueous ammonium dihydrogen phosphate electrolyte wherein the foil is not anodized in an aqueous acid electrolyte prior to forming the foil in an aqueous ammonium citrate electrolyte.